Coatings play an important-role in today's manufactured products based society. Coatings provide immunity to corrosion, thermal insulation, shielding, enhanced appearance, as well as aid in identification. Paints and other types of protective coatings are purposely designed to adhere to the substrate to which they are applied and resist removal. Thus, the removal of paints and other coatings from a substrate is usually very difficult.
During the life of many manufactured products, such as bridges, aircraft, automobiles, and ships, coatings must be removed and replaced for a variety of reasons. For example, refurbishment of the paint on aircraft is a regular maintenance item. Commercial airlines typically repaint their aircraft about every four to five years of service. Coatings on the exterior surfaces of large ships or bridges require periodic refurbishment in order to prevent or inhibit corrosion.
The removal of paint from the surfaces of aircraft presents special problems. Such surfaces are large, irregularly shaped, and relatively delicate. Because the surfaces of aircraft are typically relatively soft aluminum, aluminum alloys, or organic-based composite materials, such surfaces and the underlying substrates are particularly susceptible to damage while undergoing paint removal which could degrade their structural integrity.
Many different methods have been used to remove painted coatings. One method, the "particle medium blast" (PMB) method involves impinging the surface to be stripped with particles such as BB's, plastic media, steel shot, wheat starch, sand, even liquids such as water and frozen particles such as frozen CO.sub.2 particles. However, PMB methods sufficiently energetic to remove hardened coatings, such as paint, by themselves may damage delicate surfaces such as found on aircraft and automobiles if they are not carefully managed. For example, if the impinging particles dwell too long at one location, the impinged surface may become pitted or stress hardened. This is especially important with regard to the surfaces of aircraft since pitting or stress hardening may change the mechanical properties of the surface material. High-energy PMB methods may also deform the surface of the substrate sufficiently to mask fatigue cracks and other anomalies which, if undetected and uncorrected, could lead to catastrophic failure of the substrate. PMB may also damage putty joints often found on aircraft between surface plates. Moreover, these processes generate a large amount of particulate waste requiring costly disposal. This waste is contaminated by toxic constituents of the coating, increasing the difficulty and expense of its disposal.
Another method involves the application of chemical agents to painted surfaces in order to chemically break down the layers of paint, thereby stripping the paint away from the surface to be exposed. However, such agents may pose a risk to human health, are usually toxic, and are often not biodegradable. Overall, these types of agents are difficult and costly to dispose of because they present serious environmental problems. Government regulations are increasingly restrictive of the use of such agents.
Still other methods involve the mechanical removal of the coating from the substrate. For example, U.S. Pat. No. 4,836,858, entitled "Ultrasonic Assisted Paint Removal Method" discloses a hand-held tool which uses an ultrasonic reciprocating edge placed in contact with the surface to be stripped. Use of such tools is labor intensive and requires skilled operators. Further, control of this tool is a problem when applied to aircraft because the aircraft surface may be damaged if there is excessive tool dwell at one location.
Still other methods involve the application of radiant energy to the coating. One such system uses a flashlamp pumped laser and video frame grabber in a video controlled paint removal system in which paint is stripped from a surface using the output of the laser to ablate the paint while a video camera converts images of the surface being stripped into electronic data signals. The data signals are used to control the laser output. A processor compares the data signals with parameters stored in a memory to determine whether sufficient paint has been removed from the surface being stripped. If an insufficient amount of paint has been removed, then the laser continues to irradiate the surface. If the surface has been adequately stripped, the processor directs the laser to ablate another area. A significant problem with the video controlled paint removal system is that the amount of data which is generated and which must be processed is enormous, making real time control extremely difficult.
Furthermore, a laser powerful enough to vaporize paint requires high power due, in part, to laser pumping inefficiencies. Employment of such a powerful laser requires a large capital investment in order to provide space to operate the laser, as well as laser stops to prevent the laser beam from inadvertently escaping the work area and even the building where it is being used. Such a laser poses a serious danger to humans, who must be kept out of the area where the laser operates. Another problem with the use of lasers is the occurrence of localized "hot spots" which can damage or destroy the substrate.
Pulsed light, for example from flashlamps, has also been used to ablate coatings from their substrates. A problem with ablating solely with radiant energy is the risk of heat damage to the substrate. Moreover, ablation can result in flame and smoke which can reduce the efficiency of or damage the equipment and which may pose a hazard for personnel. Further, these processes typically leave a non-water soluble carbonized residue that must be removed in a labor-intensive process.
The use of composite structures manufactured, for example, of carbon epoxy or other reinforced plastic materials is becoming increasingly common. Many aircraft and automobiles extensively employ reinforced composite materials, including carbon epoxy materials, for surface structures. Such structures are painted for a variety of reasons including aesthetics, identification, and camouflage. However, such painted surfaces deteriorate under the action of weather and the mechanical forces to which they are subjected, thus requiring removal and replacement.
Other than hand sanding, there have been no suitable methods for removing paint from non-metallic composite surfaces. PMB and mechanical grinding methods sufficiently energetic to remove paint by themselves have proven to damage composite materials. The removal of paint with chemical agents does not offer a satisfactory solution because such chemicals tend to attack the organic binder in the composites, as well as the paint. Some composites are sensitive to high temperatures, such that high temperature methods of removing coatings are not satisfactory.
For a variety of reasons, paint removal techniques for removing paint from large surfaces, and surfaces having topological anomalies such as rivets, have not proven wholly satisfactory. Thus, it can be appreciated that coating removal, and particularly, the removal of paint from large and often delicate surfaces such as found on aircraft and automobiles, is a problem that has not heretofore been satisfactorily solved.